Conventionally, a vehicle having an internal combustion engine mounted therein has been commercialized. In such a vehicle, for example, a power supply system is mounted as shown in FIG. 5. In FIG. 5, a battery 101 that supplies electric power of a predetermined voltage (for example, approximately 12V) is mounted in the vehicle. The battery 101 is charged with electric power generated by an alternator (generator) 102 that is driven under a rotation torque from the internal combustion engine. To the inside of the vehicle, a plurality of electric loads that are supplied with electric power from the battery 101 or the alternator 102 are connected. In FIG. 5, as exemplary electric loads, an ECU 103 and a plurality of actuators 104 (three representative actuators 104a to 104c) are shown. Note that, the operations of the actuators 104a and 104b are each controlled by the ECU 103.
A power supply system with two batteries, which is mounted in a vehicle, has also been developed, the two batteries supplying electric power of different voltages, respectively (for example, see Patent Document 1). In the power supply system, upon detection of a drop of the output voltage of one battery, the output voltage of the other battery is boosted or stepped down to the output voltage of the one battery by a DC-DC converter, such that the resultant electric power is supplied.
Patent Document 1: Japanese Laid-open Patent Publication No. 2002-171691